Methods and apparatus for minimally invasive modular interbody fusion devices

ABSTRACT

The invention is a modular interbody fusion device for fusing adjacent spinal vertebrae that is adapted to be implanted in a prepared interbody space including a first modular segment having a width including a first rail extending at least partially along one side of the width and beyond a periphery of a body portion of the first modular segment, a second modular segment having a width and slidably connected to the first rail on one side of the width and having a second rail extending at least partially along another side of the width and beyond a periphery of a body portion of the second modular segment, a third modular segment having a width and slidably connected to the second rail on one side of the width and wherein the device has an expanded position in which the second and third modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation and an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a unitary body that mimics the planar shape of the vertebra such that the device contacts and supports the adjacent vertebra.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/974,185 filed Oct. 11, 2007; which claims the benefit of U.S.Provisional Application No. 60/860,329 filed Nov. 21, 2006, each ifwhich is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to an implantable orthopedicfusion device for fusing joints in a patient such as a vertebralinterbody fusion device. More particularly, the present inventionrelates to a rail-based modular interbody fusion device of predeterminedsize and shape.

BACKGROUND OF THE INVENTION

Joint fusion or arthrodesis is a common approach to alleviate the paindue to deteriorated and/or arthritic joints. Joint fusion involvesinducing bone growth between two otherwise mobile bones in a joint,which alleviates pain by immobilizing and stabilizing the joint. Thejoint is generally fused in its most functional position. The ankle,wrist, finger, toe, knee and vertebral joints are all examples of jointsthat may be fused to alleviate pain associated with unstable,deteriorated joints.

The spinal motion segment consists of two adjacent vertebral bodies, theinterposed intervertebral disc, as well as the attached ligaments,muscles, bony processes and the facet joints. The disc consists of theend plates at the surfaces of the vertebral bones, the soft inner core,called the nucleus pulposus and the annulus fibrosus ligament thatcircumferentially surrounds the nucleus and connects the vertebraetogether. In normal discs, the nucleus cushions applied loads, thusprotecting the other elements of the spinal motion segment. The nucleusin a normal disc responds to compression forces by bulging outwardagainst the vertebral end plates and the annulus fibrosus. The annulusconsists of collagen fibers and a smaller amount of elastic fibers, bothof which are effective in resisting tension forces. However, the annuluson its own is not very effective in withstanding compression and shearforces.

As people age the intervertebral discs often degenerate naturally.Degeneration of the intervertebral discs may also occur in people as aresult of degenerative disc disease. Degenerative disc disease of thespine is one of the most common conditions causing back pain anddisability in our population. When a disc degenerates, the nucleusdehydrates. When a nucleus dehydrates, its ability to act as a cushionis reduced. Because the dehydrated nucleus is no longer able to bearloads, the loads are transferred to the annulus and to the facet joints.The annulus and facet joints are not capable of withstanding theirincreased share of the applied compression and torsional loads, and assuch, they gradually deteriorate. As the annulus and facet jointsdeteriorate, many other effects ensue, including the narrowing of theinterspace, bony spur formation, fragmentation of the annulus, fractureand deterioration of the cartilaginous end plates, and deterioration ofthe cartilage of the facet joints. The annulus and facet joints losetheir structural stability and subtle but pathologic motions occurbetween the spinal bones.

As the annulus loses stability it tends to bulge outward and may developa tear allowing nucleus material to extrude. Breakdown products of thedisc, including macroscopic debris, microscopic particles, and noxiousbiochemical substances build up. The particles and debris may producesciatica and the noxious biochemical substances can irritate sensitivenerve endings in and around the disc and produce low back pain. Affectedindividuals experience muscle spasms, reduced flexibility of the lowback, and pain when ordinary movements of the trunk are attempted.

Degeneration of a disc is irreversible. In some cases, the body willeventually stiffen the joints of the motion segment, effectivelyre-stabilizing the discs. Even in the cases where re-stabilizationoccurs, the process can take many years and patients often continue toexperience disabling pain. Extended painful episodes of longer thanthree months often leads patients to seek a surgical solution for theirpain.

Several methods have been devised to attempt to stabilize the spinalmotion segment. Some of these methods include: applying rigid orsemi-rigid support members on the sides of the motion segment; removingand replacing the entire disc with an articulating artificial device;removing and replacing the nucleus; and spinal fusion involvingpermanently fusing the vertebrae adjacent the affected disc.

Spinal fusion is generally regarded as an effective surgical treatmentto alleviate back pain due to degeneration of a disc. The fusion processrequires that the vertebral endplates be prepared by scraping thesurface of the existing vertebral bone to promote bleeding and releaseof bone growth factors, and placing additional bone or suitable bonesubstitute onto the prepared surface. Devices of an appropriate sizemade from rigid materials such as metals (including titanium andtantalum), some plastics (including polyetheretherketone (PEEK), orcarbon fiber-filled PEEK), and allograft bone (primarily from donorfemurs) are commonly inserted into the prepared disc cavity as part ofthe interbody fusion procedure to help distract and stabilize the discspace and put the vertebra into proper position while the bone growthprocess takes place. The interbody fusion procedure may be accomplishedfrom an anterior, transforaminal, or a posterior surgical approach.

Most devices used in interbody spinal fusion require a relatively largeopening that is typically larger than the dimensions of the rigid andunitary fusion device or cage that is to be inserted, examples of suchdevices include, U.S. Pat. No. 5,026,373 to Ray et al., U.S. Pat. No.5,458,638 to Kuslich et al., and the NOVEL™ PEEK Spacers from Alphatec.In fact, many methods of interbody fusion, for example the method anddevice described in U.S. Pat. No. 5,192,327 to Brantigan, requirebilateral placement of unitary devices through fairly large surgicalopenings. As with any surgical procedure, the larger the surgical accessrequired, the higher the risk of infection and trauma to the surroundinganatomy.

There exists minimally invasive spinal fusion devices such as isdisclosed in U.S. Pat. No. 5,549,679 to Kuslich and U.S. Pat. No.6,997,929 to Manzi et al. The device disclosed in the U.S. Pat. No.5,549,679 is a porous mesh bag that is filled in situ. The U.S. Pat. No.6,997,929 is directed to a series of wafers that are vertically stackedto distract and support the vertebral endplates. U.S. Pat. No. 5,702,454to Baumgartner discloses plastic beads which may be inserted one at atime into an intervertebral space on a flexible string. Further, U.S.Pat. No. 5,192,326 to Bao discloses hydrogel beads encased in asemi-permeable membrane.

While such minimally invasive technologies permit smaller accessincision through the annulus (i.e. an annulotomy) to be used in a fusionprocedure, the resulting fusion devices do not have the mechanical anddimensional features of the more rigid unitary fusion devices used intraditional surgical approaches and are less able to distract andstabilize the disc space. Thus, there is a need for a minimally invasivespinal fusion implant that could better emulate the mechanical andstructural characteristics of a rigid unitary fusion device.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for a rail-basedmodular interbody fusion device having a predetermined size and shapewhen assembled in situ. In one embodiment, the modular interbody fusiondevice comprises generally solid modular segments with rails thatoperably connect adjacent modular segments. This configuration allowsthe interbody spacer to be adapted for implantation via a small accessincision or annulotomy through various surgical approaches, including aposterior or a lateral approach. In one embodiment, the rails operatewith a sliding mechanism to connect and interlock adjacent modularsegments. A stem portion of the rails that extends beyond the peripheryof the body of the prosthesis is removable after implantation such thatthe modular segments combine to form a single device with a relativelysmooth outer circumference when assembled in situ. The modular fusiondevice can be configured to provide full contact with and closely mimicthe geometry of the surfaces of the joint being fused so as to moreclosely mimic the functionality of the largest existing rigid andunitary fusion devices.

In one embodiment, an interbody modular fusion device is adapted to beimplanted in a prepared intervertebral space and includes at least threemodular segments each having a width. The first modular segment has afirst rail extending at least partially along one side of the width andbeyond a periphery of the first modular segment. The second modularsegment is slidably connected to the first rail on one side of the widthand has a second rail extending at least partially along another side ofthe width and beyond a periphery of the second modular segment. Thethird modular segment is slidably connected to the second rail on oneside of the width. The interbody fusion device has an expanded positionin which the modular segments are extended along the first and secondrails and positioned in a generally end to end configuration spacedapart by the rails prior to implantation. The interbody fusion devicealso has an implanted position in which the modular segments arepositioned in a generally side by side configuration that defines asingle assembled body having a generally continuous periphery thatgenerally corresponds to the inner boundary of the annulus.

In one embodiment, each modular segment has a compressive modulus in thesuperior to inferior direction from about 0.5-15 GPa, such that thecompressive modulus of the interbody fusion device generally correspondsto the compressive modulus of the surrounding cortical bone.

In one embodiment, locking features are provided to ensure that themodular interbody spacer is a unitary device both before and afterinsertion. To prevent the device from being separated prior toinsertion, locking features may be provided on the rigid rails toprevent modular segments from being slid back off of the rails. Thisensures that each modular segment is connected in its proper positionand in the proper order. In addition, locking features may be providedon the modular segments to lock them together upon insertion. Thisprevents individual segments from dislocating from the assembledprosthesis and migrating outside of the annulus. Further, the interbodyfusion device may include grooves, ridges, or other structures on itsouter surface to contact surrounding bone and prevent the device frommigrating out beyond the anterior limit of the intervertebral space.

Another aspect of the present invention comprises a method forimplanting an interbody spacer. Because the modular interbody spacer maybe implanted one segment at a time, a hole made in the annulus forimplantation of the prosthesis may be a fraction of the size of thedevice in its final assembled form. The first modular segment isinserted into the intervertebral space through the small hole in theannulus. The second modular segment is then slid up the first rigid railand into the intervertebral space until the second modular segmentinterlocks with the first modular segment. The tail stem of the firstrigid rail is then severed from the device. This severing may beaccomplished by simply snapping the rail off the device. Alternatively,the tail stem may be attached to the device by a screw, a bayonetmechanism, a twist lock or the like. As such, the rails may be removedfrom the device by unscrewing, or releasing the bayonet, etc. Subsequentmodular segments are slid up the adjoining rigid rail into the interbodyspace and then interlocked with the previously inserted modular segmentin a similar manner. Once all of the modular segments have been insertedand all of the tail stems severed, the modular interbody spacer is fullyinserted into the patient's interbody space.

Another aspect of the present invention provides an insertion tool thatmay be used to aid in the insertion, positioning, and rail removal ofthe modular interbody spacer. The proximal end of the tool has a handlewith an enclosed ratchet or roller mechanism attached to and in linewith the inner lumen of an elongated tube at the distal end of the toolthrough which a rail may be inserted. The elongated tube may have a slitor other openings along the length of the tube to aid in threading therails into the tube. The insertion tool may be provided with a cuttingmechanism for removing the rails from the modular segments once they arefully inserted.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a top view of a modular interbody spacer according to anembodiment of the present invention in its inserted configuration.

FIG. 2 is a perspective view of a modular interbody spacer according toan embodiment of the present invention at a first stage of insertion.

FIG. 3 is a perspective a view of a modular interbody spacer accordingto an embodiment of the present invention at a second stage ofinsertion.

FIG. 4 is a perspective view of a modular interbody spacer according toan embodiment of the present invention at a final state of insertion.

FIG. 5 is a perspective view of an alternate embodiment of the device.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there can be seen a top view of a modular interbodyspacer 100 according to an embodiment of the present invention asconfigured once inserted into the body. In this embodiment, modular discprosthesis 100 comprises first 102, second 104, third 106, and fourth108 modular segments. Interbody spacer 100 may be comprised of anysuitable biomaterial, for example, a polymer, such as PEEK, a metal,such as titanium, trabecular metal, bone, or a resorbable material thatmay act as a scaffold for new bone growth and/or a carrier for stemcells.

Modular segments 102, 104, 106 and 108 may be inserted via a smallannulotomy from a posterior or lateral approach. Interbody spacer 100may then be constructed within the interbody space by first insertingmodular segment 102 into the interbody space, then sliding modularsegments 104, 106 and 108 along a series of rails wherein each segmentlocks with the previous segment to create an interbody spacer 100 havinga final, assembled surface area that fully contacts and supports thevertebral end plates.

Interbody spacer 100 may include locking barbs that prevent individualunits from backing out or extending beyond the anterior limit of thespacer. Spacer 100 may further include grooves, ridges 142 or otherstructures to engage the surrounding bone or otherwise prevent spacer100 from backing out of the intervertebral space.

In a preferred embodiment, interbody spacer 100 may be made of PEEKhaving holes therethrough, allowing for tissue ingrowth thus promotingbony fusion. The holes 140 may be of varying size and shape. Holes 140may be spaced apart on spacer 100 in any manner such that thecompressive modulus of spacer 100 generally corresponds to thecompressive modulus of the adjacent bone. Spacer 100 may also be ofvarying thicknesses to achieve the desired support and/or fusion of aparticular intervertebral space, such as a lordotic configuration forL5-S1 fusion.

In an embodiment, prior to insertion, holes 140 of interbody spacer 100may be packed or filled for example with, autologous bone graft,calcified or decalcified bone derivative, bone graft substitute, such ashydroxyapatite, agents to promote bone growth, such as bonemorphogenetic protein, or osteogenic protein-1, antibiotics, anti-canceragents, stem cells, biologically active cytokines, cytokine inhibitors,fibroblast growth factors, other osteoinductive and/or osteoconductivematerials or any other material and combination thereof to promotefusion and/or stabilize the spinal motion segment.

In another embodiment, interbody spacer 100 may include surfacemodifications to provide for elution of medicants. Such medicants mayinclude analgesics, antibiotics, anti-inflammatories, anticoagulants,antineoplastics or bioosteologics such as bone growth agents. In analternative embodiment, spacer 100 may be comprised of a material, suchas for example, porous PEEK, from which an imbibed medicant can elute.In yet another embodiment, an inner portion of the spacer 100 may becomprised of one material, while the outer portion is comprised ofanother material. For example, the inner portion may be comprised of asolid PEEK, while the outer portion is comprised of a porous PEEK. Thesurface of the porous PEEK may be coated with a bioactive agent ormedicant. Spacer 100 may be imbedded with a radiopaque material, such astantalum or titanium beads to allow for x-ray visualization of theimplant.

In another embodiment, the rails may be used as fill tubes such thatfill material may be injected or otherwise inserted into holes 140.Spacer 100 may also be manufactured to include channels or ducts intowhich fill material may be inserted via the rails.

Referring to FIG. 2, there can be seen a portion of a modular interbodyspacer 100 according to the preferred embodiment of the presentinvention prior to insertion into the intervertebral space. In alternateembodiments, the modular interbody spacer may comprise greater or fewernumbers of modular segments and rails.

Prior to insertion, modular interbody spacer 100 further includes first110, second 112, and third 114 rails. First modular segment 102 isrigidly attached to first rail 110 at first segment interlocking portion116. As shown in FIG. 3, second modular segment 104 is slidably attachedto first segment interlocking portion 116 at first slot 128 and rigidlyattached to second rail 112 at second segment interlocking portion 118.As shown in FIG. 4, third modular segment 106 is slidably attached tosecond interlocking portion 118 at second slot 130 and rigidly attachedto third rail 114 at third segment interlocking portion 120. Fourthmodular segment 108 is slidably attached to third rail 114 at fourthslot 133.

As shown in FIG. 2, each rail 110, 112 and 114 includes a stem portionthat extends beyond a periphery of the body of the spacer 100,respectively. Preferably these stem portions are long enough to permitaccess into the intervertebral space such that one modular segment canbe positioned inside the intervertebral space while the next modularsegment on the rail is still outside of the body. In an exemplaryembodiment, the length of the stem portions ranges between 6 cm-20 cm.Each rail 110, 112 and 114 may further include a retaining portion tokeep the device from being separated prior to insertion. The retainingportions are configured to prevent the corresponding modular segmentsfrom sliding off the rails. The retaining portions may be molded intothe rails or may be separate pieces or deformations of the rails addedduring the manufacture of the device. Rails 110, 112, 114 may besequentially removed from the implant as modular segments 102, 104, 106,and 108 are connected within the intervertebral space and movedlaterally.

The preferred embodiment is an interbody spacer that is packaged,sterile, and ready for implantation at the surgical site. The packagemay include any number of modular segments. In a preferred embodiment,the package would include 5 individual modular segments. Single modulepackages may also be used so that the surgeon may use as many segmentsas desired. Since the device is fully preformed and delivered as aunitary implant, the device is under direct surgeon control until theinterbody spacer is completely formed. This unitary design reduces theneed for the surgeon to determine how to configure the spacer to allowfor the most efficacious placement of the spacer in the intervertebralspace and assures that the components' order of insertion and connectionare properly achieved. The size and shape of the modular interbodyspacer provides a final, assembled surface area that fully contacts andsupports the vertebral end plates, stabilizing the spinal unit. In thisregard, it will be understood that the modular interbody spacer 100 ofthe present invention may be provided in a variety of different finalassembled sizes to correspond to different sizes of differentintervertebral spaces.

In an alternative embodiment as shown in FIG. 5, separate guide rods 150and a guide mechanism 152 may be used to assist in inserting andaligning the modular segments. Rod 150 may be attached to the proximalend of each modular segment. Rod 150 may be used to insert a firstmodular segment into position. A second guide rod may be attached to asecond modular segment and used to place the second modular segment inposition to mate and interlock with the first modular segment. The firstrod could then be detached. Subsequent segments could be inserted byrepeating the process.

In an embodiment, a modular segment may include a tapped hole 154 suchthat rod 150 may be screwed into hole 154. Rod 150 does not participatein the interlocking mechanism of modular segments. In an embodiment, rod150 may either be made of the same material as the modular segments, orrod 150 may be comprised of a different material, including, but notlimited to, plastics such as PEEK, or metals such as stainless steel ortitanium. According to one aspect of the present invention, rod 150 maybe integral to the modular segments. For example, rod 150 may beinjection molded from a plastic or machined from a plastic or metal.

In another embodiment of the present invention, rod 150 may be formedseparately from the modular segments and then joined to the modularsegments via a mechanical method such as a mating thread, twist-lock,snap-lock or such, or by the use of adhesives or other material joiningmethods such as thermal and ultrasonic welding. One advantage to using amechanical method of joining rod 150 to the modular segments is thepotential to re-engage the modular segments for removal from the discspace, should the need arise. The removal sequence of rods 150 from themodular segments following implantation of the modular segments in thedisc space is the same as for interlocking rails.

In an embodiment, modular interbody spacer 100 may be introduced throughan access tube that is inserted partially into the intervertebral space.The access tube is at least 3 inches long and preferably about 6 incheslong. It should be noted that although the insertion of modularintervertebral spacer 100 is described in relation to a four-segmentembodiment, embodiments having any other number of segments would beinserted in a similar fashion.

During insertion, slots 128, 130, 133 slide along the stem portions ofrails 110, 112, 114 and onto segment interlocking portions 116, 118,120. Slots 128, 130, 133 and segment interlocking portions 116, 118, 120may be provided with locking features to prevent separation of modularsegments 102, 104, 106 and 108. Locking features, such as a barb or studor a series of barbs or studs, may be provided such that once a slot isslid onto a segment interlocking portion, it cannot be slid back off ofit. A ratchet and pawl may also be used to lock modular segmentstogether. A ratchet release tool may also be provided in case separationof modular segments is desired once they are locked together.

Various modifications to the disclosed apparatuses and methods may beapparent to one of skill in the art upon reading this disclosure. Theabove is not contemplated to limit the scope of the present invention,which is limited only by the claims below.

The invention claimed is:
 1. A modular interbody fusion device forfusing adjacent spinal vertebrae that is adapted to be implanted in aprepared interbody space, the device comprising: a first modular segmenthaving a first width including a first rail extending at least partiallyalong one side of the first width and beyond a first periphery of afirst body portion of the first modular segment; a second modularsegment having a second width and slidably connected to the first railon one side of the second width and having a second rail extending atleast partially along another side of the second width and beyond asecond periphery of a second body portion of the second modular segment;a third modular segment having a third width and slidably connected tothe second rail on one side of the third width; and an insertion toolfor sliding the second modular segment along the first rail and thethird modular segment along the second rail, wherein the device has anexpanded position in which the second and third modular segments areextended along the first and second rails and positioned in a generallyend to end configuration spaced apart by the rails prior to implantationand an implanted position in which the modular segments are positionedin a generally side by side configuration that defines a unitary bodyadapted to contact and support adjacent vertebra, wherein the insertiontool includes a separation mechanism to remove the first rail and thesecond rail following assembly of the unitary body, and wherein each ofthe first, second and third modular segments includes at least one holetherethrough, said holes adapted for tissue ingrowth from the adjacentspinal vertebrae such that the unitary body is adapted to support andstabilize fusion of the adjacent spinal vertebrae.
 2. The modular fusiondevice of claim 1 wherein one or more of the holes may be packed withfill material.
 3. The modular fusion device of claim 2 wherein the fillmaterial is selected from the group consisting of: bone graft, bonegraft substitute, agents to promote bone growth, antibiotics,anti-cancer agents, stem cells, fibroblast growth factors, bonederivative, biologically active cytokines, cytokine inhibitors,osteoconductive material, osteoinductive material and any combinationthereof.